Atheroma or atherosclerosis corresponds to a rearrangement of the intima of large- and average-sized arteries (aorta, coronary arteries, cerebral arteries, low limber arteries, etc.) by segmental accumulation of lipids, complex carbohydrates, blood and blood products, fatty tissues, calcareous deposits, and other minerals. This vascular pathology generally has a slow progression (over decades). It may stabilize and not represent a significant danger for the patient. But it may also degenerate into an instable form leading to a rupture of the plaque, and, within a few days, and cause lethal or morbid cardiovascular or cerebral accidents (CVA).
Indeed, the rupture of a plaque brings its contents into contact with the blood conveyed by the artery, which can result in the formation of a thrombus. The latter disturbs the bloodstream in the affected artery. It can also detach and be transported by the bloodstream, and, in the most severe cases, totally obstructs the lumen of the artery, stop the blood supply of the post-lesion region and lead to the ischemia thereof.
Tissue characterisation is of fundamental interest in medical diagnosis, especially for estimating a rupture risk of an atheromatous plaque. For the last twenty years, a new medical imaging method has been developed. It is the ultrasonic elastography.
Based on the same principles as palpation, elastography locally studies the elastic behaviour of medium under the action of a stress. This study is based on the analysis of radiofrequency ultrasonic signals acquired before and after applying a stress, or acquired for different levels of stress.
As indicated above, the atherosclerosis plaque implies the deposit of lipids and/or collagen on the vessel walls. This deposit results in an increased or decreased elasticity of the vessel walls. The ultrasonic elastography provides the practitioner information enabling him/her to estimate the rupture risks of an atheromatous plaque.
EP 0 908 137 describes an ultrasonic elastography method enabling an image of the elastic characteristics of the medium to be provided. More precisely, this method enables a local apparent “stiffness” of the endoluminal thick layer of a cavity of a body such as an artery to be determined and displayed. However, this method has major drawbacks questioning the credibilities (i.e. they are not real elasticities) of the formulations of local and global elasticity respectively described in EP 0 908 137.
Especially, the local elasticity of the endoluminal thick layer of the cavity of the body is estimated assuming that:                the inner and outer walls of the body are cylindrical and concentric (i.e. uniform thickness of the body between the inner and outer cylindrical walls of the cavity),        both inner and outer walls of the body are subjected to uniform spatial pressure distributions,such that the elasticity values supplied by implementing this method are very different from the real elasticity values of the analysed body and thus have no physical meaning (i.e. this is not a real elasticity).        
Thus, the method described in EP 0 908 137 does not enable the practitioner to have sufficiently accurate available information to carry out a diagnosis. One purpose of the present invention is to provide an ultrasonic elastography method enabling the drawbacks of the method described in EP 0 908 137 to be overcome.